High frequency electric induction heating systems



@ch 14, 1958' D. s. JONES HIGH FREQUENCY ELECTRIC INDUCTION HEATINGSYSTEMS Filad Oct. 24. 1955 ZSheets-Sheet 1 lNvEN-row.

DAv lu GRIFFITHS Jones .BYW W ATTORNEYS Oct. 14, 1958 v D. e. JONES2,855,493

HIGH FREQUENCY ELECTRIC mnucnon HEATING SYSTEMS Filed Oct. 24, 1955 2Sheets-Sheet 2 \NVENTOR DAym Gamrrrus Jauss A! ATTORNEYS iijnite StatesPatent HIGH FREQUENCY ELECTRIC INDUCTION HEATlNG YSTEMS David GriffithsJones, Cheltenham, England, assignor to Deiapena dz Son Limited,Cheitenham, England Application October 24, 1955, Serial No. 542,395

Claims priority, application Great Britain November 30, 1950 7 Claims.(Cl. 21910.75)

This invention relates to high frequency electric inducticn heatingsystems and has for one of its objects to provide a. high frequencyinduction heating system the output of which can be varied by meanswhich are controlled manually or automatically and having the advantagethat when controlled automatically there is no appreciable delay in thedesired variation of the output.

Another object of the invention is to provide a high frequency inductionheating system in which variation of the output is effected by varyingthe direct current flowing through a control coil mounted on themagnetic core of a transformer of the system, which variation alters theeffective permeability of the magnetic core and thereby varies the fluxlinkage between primary and sec ondary windings of the transformer,increase in the supply of direct current to the control coil loweringthe permeability of the magnetic core and in consequence decreasing theoutput of the induction heating system and vice versa.

By means, therefore, of a single transformer incorporated in the highfrequency induction system the output of the latter, which is fed to awork coil, can be varied to suit the requirements of a workpiece beingheated by the work coil or maintained at a substantially constant level.This application is a continuation-inpart of my application Serial No.258,969, filed November 29, 1951, now abandoned.

The supply of direct current for the control coil may be obtained eitherfrom a circuit to which the transformer input or output is connected,the required direct current being supplied through a rectifier, oralternatively an independent direct current source may be provided.

In operation, when no direct current is supplied to the control coil theprimary and secondary windings of the transformer are closely linked bythe flux in the magnetic core so that the transformer output is as highas possible in relation to the input. As direct current is passedthrough the control coil the effective permeability of the magnetic coreis reduced and consequently the flux linkage between the windings isreduced with the result that the value of the transformer output isreduced.

The output from the induction heating system is, therefore, dependent onthe value of the direct current in the control coil and can be increasedor decreased according to, respectively, a decrease or increase of thedirect current supplied to the control coil.

Due to the rapidity with which the output of the induction heatingsystem can be controlled treatment of articles requiring only extremelyshort heat treatment, for example one or two seconds, can besatisfactorily effected. Thus the means employed for controlling theoutput current embodied in the present invention have a considerableadvantage over the conventional means for varying the current ininduction heating systems as such means involve the use of mechanicallymoving 2,856,498 Patented Oct. 14, 1958 devices which cannot be operatedwith sufficient rapidity to effect control during short periods ofheating.

A still further object of the invention is to provide a high frequencyinduction heating system which enables adjustment of the output currentfed to the work coil to be made to suit the particular requirements ofthe article being heated whilst maintaining the output current of thegenerator constant, enables compensation for changes in supply mainsvoltage to be made, or the maintenance of a high level of output powerfrom the generator to be effected when the physical characteristics ofthe material being heated alter with temperature.

An important example of the last-mentioned application is in heatingmagnetic materials such as steel, where the power absorbed from the workcoil by the material, when the latter is heated above the Curietemperature, is very much less than it is when such material is belowthe Curie temperature. If the output current is increased as thetemperature of the work rises above the Curie temperature then the powerabsorbed by the material being heated may be kept at a high level.

Other objects and advantages of the present invention will becomeapparent from the following detailed de scription of two specificembodiments thereof, when taken in conjunction with the accompanyingdiagrammatic drawings, in which:

Figure 1 illustrates a high frequency induction heating system includinga generator in which an oscillator valve is incorporated, a transformerhaving a control coil fed with direct current from the source supplyingthe anode of the oscillator valve, and a work coil fed from thesecondary windings of the transformer,

Figure 2 illustrates an alternative arrangement in which the controlcoil is fed from a rectifier connected across the secondary windings ofthe transformer,

Figure 3 shows the winding arrangements of the transformer illustratedin Figures 1 and 2, and

Figure 4 is a cross-sectional view of one of the windings of thetransformer.

Referring to Figures 1 and 3 of the drawings, a generator for highfrequency current is indicated generally at 27, such generator includingan oscillator valve 28. A high tension rectifier 29 fed with currentfrom the mains 30 supplies positive direct current to the anode 31 ofthe oscillator valve 28 through a lead 32. High frequency current fromthe generator 27 is fed to a transformer, separately illustrated inFigure 3, having a core 10 which in the example shown is of laminatedconstruction and is built up from thin strip magnetic material, althoughthe core may be otherwise constructed from magnetic core materialsuitable for use at radio frequencies. Such core is of rectangular shapeto provide a closed iron circuit and is provided, as shown in Figure 3,with three parallel limbs of which two 11, 12 are constituted by theopposite end members of the rectangle while the third 13 is positionedcentrally of the end members. The outer limbs 11, 12 form the transverseportions of U-shaped members 14, 15, the limbs 14a, 15a of such membersextending inwardly towards each other so that their free ends abut theopposite ends of the central limb 13 of the core. Thus the threeparallel limbs 11, 12, 13 of the core are connected together at oppositeends by parallel side members formed by the limbs 14a, 15a of theU-shaped members.

The primary windings of the transformer consist of two similar coils 16,17 mounted respectively on the outer limbs 11, 12 of the core, the coilsbeing wound in opposite directions upon the respective limbs andconnected in series as shown. A similar arrangement is provided for thesecondary winding which comprises two coils 18, 19 arranged in seriesand mounted respectively on the outer limbs 11, 12. Thus the windingsare arranged on the core 10 with a primary and secondary coil on each ofthe outer limbs thereof. .A control .coil 20 is wound around the thirdor central limb 13 of the core and is adapted to be connected to asuitable source of direct current.

In the embodiment shown in Figure l 'the primary windings 16, 17, whichtogether with parallel condensers form the tank circuit inductance ofthe valve oscilla'tor, are fed with current from the generator .27whilst the output from the secondary windings 18, 19 is fed to a workcoil 33. By means of a lead 3% direct current from the cathode .35 ofthe oscillator valve 28 is fed to the control .coil 20 of thetransformer through a filter circuit 36. The opposite end of the controlcoil 20 is connected by a lead 37 to the negative side 29a of therectifier .29.

During operation of the induction heating system positive high tensioncurrent flows from the rectifier '29 through the lead :32 to the anode31, from the latter to the cathode 35 and thence through the lead 34 andcontrol coil 20 to the negative side 2% of the rectifier. Whenabsorption of power by the article being heated by the work coil 33 fromthe secondary windings 18, 19 of the transformer is reduced, withconsequential reduction in the current flowing through the primarywindings 16, 17 and from the anode 31 to the cathode 35 of theoscillator valve 28, the direct current flowing through the control coil20 is also reduced with the result that the output of thesecondarywindings 18, 19 of the transformer is increased. In consequenceof this arrangement, as the rectifier current decreases when theabsorption of power fro-m the output is reduced, for example when steelbeing heated rises above the Curie point, the output current fed 'to thework coil 33 by the secondary windings 18, 19

'is increased, and soincreases the effective output power of thegenerator. A'variable resistance 38 is arranged in parallel "with thecontrol-coil -20 to enable adjustment of theamount of current flowingthrough the latter.

If it is desired .to'maintain the output current constant, the controlcurrent may be ob'tained by rectifying 'aismall current obtained'fromaheoutput voltage. -An increase'in the output current will then supplyan increased control current through the control coil 20, which willreduce the flux linkage between input and output circuits, i. e. betweenthe primary and secondary windings 16, 17; 18, 19, and tend to keep theoutput current constant. Such an arrangement is i-sh'own in Figure 2which illustrates a circuit in which, as in theprevious embodiment, highfrequency current is supplied to the primary windings 16, 17 'by agenerator 27. A full wave rectifier 39 is connected .by leads'40, 41across the output of the secondary windings 18, .19 mounted on the core10 of 'the transformer. Direct current from the output side of therectifier 39 is fed through leads 42, 43 to the control coil 20, avariable resistance 44 being incorporated in the lead 43 to controlthe'proportion of the output current of the transformer which is fedthrough the control coil. An increase in the output current from thesecondary windings 18, .19'williresult in an increase in the currentpassing through the'rectifier-39 and control coil 20 which will reducethe flux linkagebetween the primary and secondary windings 16, 1-7; 18,19 and tend to keep the output fed to the work coil 33 constant.

Alternatively, for manual control the value of the control current maybe determined by a variable resistance 21 inserted in one of the twoleads 22, 23 by which direct current is supplied to the control coil 20from any suitable source of electrical energy. If it is not desired toutilise the direct current of the generatorof the induction heatingapparatus the leads'22, 23 may, for example,

'be connected to the output terminals of a small rectifier.

In order to minimise the pick-up of radio or high -frequency voltage inthe control coil "20 the core of' the transformer and the primary andsecondary 'windings'1'6,

17; 18, 19 thereof should be constructed as symmetrically as possible.It will be appreciated that, as the primary windings 16, 17 are woundabout the respective limbs 14-, 15 in opposite directions, theinstantaneous magnetic fluxes induced in the central limb 13 by eachprimary winding flow through such limb simultaneously in oppositedirections so that these magnetic fluxes cancel each other 'and do notinduce alternating currents in the control coil 20. To ensure additionalstability in the direct current supply to the control coil, aninductance capacity filter 24 is included in the leads 22, 23 to thecoil 20 so as to prevent any induced radio or high frequency voltagebeing fed back into the control circuit.

It has been found in practice that, when the core 10 is of laminatedconstruction, for radio frequency induction heating systems a suitablethickness for the laminae is .002 in., whilst for normal high frequencysystems a suitable thickness is .007 in. Where butt joints are formed inthe core, such as those formed at 25 by the abutting ends and sides ofthe U-shaped members 14, 15 and the central limb 13, the abutting facesof the core should be ground to provide smooth mating surfaces and caremust be exercised during such grinding operation to avoid burring theends of the laminae.

in spite of the fact that the materials from which the core isconstructed have a low loss factor and that the laminae are formed fromthin material, a considerable amount of heat is generated in thetransformer and 'has to bedispersed. Losses'due to such heat generation,

as -well'as the size and consequent cost of the transformer,

which contains oil in which the transformer and its "windings-aresubmerged, the leads to such windings pass- 'ing through the sides ofthe tank. As shown in Figure *4, either the primary winding 16, 17 orthe secondary winding 18, 19, or both of such windings, are constructedof-copper tube through which water is passed for cooling purposes. Incircuits in which the primary and secondary windings have a common endconnection the water circuits through the windings may be connected inseries. Heat generated in the windings is removed directly therefrom bythe water the temperature of which rises on'ly slightlyduring itspassage through the wind- 'ings, whilst heat generated in the laminatedcore through hysteresis and eddy current is transmitted to the-oil inthe 'tank 26. As soon as the temperature of the oil exceeds that of thewater passing through the windings of the trarisformerheat istransferred from the oil through the copper tubes forming the windingsto the water and is conducted away thereby. To ensure adequatecirculation of-the' oil around the entire surface of the core 10 :thewindings should'be well spaced therefrom but this is not disadvantageoussince the high voltages normally used demand adequate spacing'betweenthe windings and the'corc.

I'claim:

1. An induction heating system including a generator for high frequencycurrent, at least one oscillator valve for said generator, aH. T.rectifier which feeds the anode of said oscillator valve, a transformerhaving a closed iron circuit and provided with three limbs, two primarywindings wound in opposite directions and arranged one on each outerlimb,two secondary windings wound in opposite directions and -arrangedone on each outer limb, a single control coil wound about the centrallimb of the transformer'toreceive direct current, andrneans for feedingcurrent from said H. T. rectifier to said control coil, whereby as thecurrent of the generator decreases with decrease in the absorption ofpower from its output the current flowing through the control coil ofthe transformer also decreases and the output thereof is increased withconsequential increase in the effective output power of the generator.

2. A high frequency electric induction heating system comprising incombination a generator for high frequency electric current, a singletransformer comprising a magnetic core, primary and secondary windingsarranged on said core, a single control coil mounted on said core toreceive a controlling direct current, a work coil to which highfrequency current is fed by said secondary windings of the transformerthe primary windings of which are energised by said generator, and meansfor varying the supply of direct current fed to said control coil whichvariation alters the effective permeability of the magnetic core andthereby varies the flux linkage between the primary and secondarywindings, increase in the supply of direct current lowering thepermeability of the magnetic core and in consequence decreasing theoutput of the induction heating system and in the current flowingthrough said work coil and vice versa.

3. A high frequency electric induction heating system comprising incombination a generator for high frequency electric current, a singletransformer comprising a magnetic core structure, a primary windingarranged on said core structure and energised by said generator, asecondary winding also arranged on the core structure, a single controlcoil mounted on the core structure to receive a controlling directcurrent, a work coil to which high frequency current is fed by saidsecondary winding, and means, comprising electric circuits which includea rectifier, for variably supplying a portion of the generator output tosaid control coil, such portion being converted to direct current bysaid rectifier.

4. A high frequency electric induction heating system comprising incombination a generator for high frequency current, an oscillator valvefor said generator, 8. high tension rectifier which feeds the anode ofsaid oscillator valve, a single transformer comprising a magnetic core,primary windings arranged on said magnetic core and fed with currentfrom said oscillator valve, secondary windings arranged on said magneticcore, a work coil to which current is supplied by said secondarywindings, a single control coil for said transformer arranged on saidmagnetic core, and means for feeding current from said high tensionrectifier to said control coil, whereby as the current of said generatordecreases with decrease in the absorption of power from its output thecurrent flowing through the control coil of said transformer alsodecreases and the output of the transformer is increased withconsequential increase in the efiective output power of the inductionheating system and in the current fed to the work coil.

5. A high frequency induction heating system comprising a singletransformer having a closed iron circuit and provided with three limbs,two primary windings arranged one on each outer limb of the transformer,two secondary windings arranged one on each outer limb of thetransformer, a single control coil wound about the central limb of thetransformer, means for variably feeding direct current to the controlcoil, a valve oscillator the tank circuit inductance of which comprisessaid primary windings, and a work coil which is fed by said secondarywindings, the inductive coupling between said primary and secondarywindings, and therefore the current fed to the work coil, being variedby varying the direct current flowing through said control coil.

6. In a high frequency induction heating system, a single transformerhaving an iron circuit comprising a core of substantially rectangularshape provided with three limbs two of which are constituted by theopposite end members of the rectangle while the third limb is positionedcentrally of and parallel to the end members, and parallel side memberswhich connect the three limbs of the core together at opposite ends, twoprimary windings wound in opposite directions and arranged one on eachof said end members, two secondary windings wound in opposite directionsand arranged one on each of said end members, a single control coilwound on said central limb to receive direct current, means for variablyfeeding direct current to the control coil to vary the inductivecoupling between said primary and secondary windings and therefore thecurrent fed to the control coil, a valve oscillator the tank circuitinductance of which comprises said primary Windings, and a work coil fedfrom said secondary windings.

7. A high frequency induction heating system according to claim 5 andfurther comprising a vessel by which the 0 transformer and its windingsis surrounded and adapted to contain oil in which the transformer isimmersed, at least one of the windings of the transformer beingconstructed of tubing through which a liquid cooling medium flows tocool such winding, the heat generated in the transformer beingtransferred to the oil which in turn is cooled by transference of heattherefrom to the liquid cooling medium flowing through the winding.

References Cited in the file of this patent UNITED STATES PATENTS1,797,268 Lee Mar. 24, 1931 1,987,730 Cravath Jan. 15, 1935 2,481,644Callaway Sept. 13, 1949 2,678,419 Bennett May 11, 1954 2,734,164Knowlton Feb. 7, 1956 2,748,241 Mohr May 29, 1956 2,748,356 Kaehni May29, 1956

